Method for inducing shear waves in the earth and device therefor

ABSTRACT

Method for inducing shear waves in the earth, comprising a generator producing two equal and opposed forces and means for transmitting said forces horizontally to vertical sections of the ground through the intermediary of springs of different rigidities, whereby said forces become unbalanced and the resultant force is transmitted horizontally to the ground.

ilite tates mm 11 1 1111 31 1 1 Lavergne 1 1 Feb. 13, 1973 54] METHODFOR KNDUCING SHEAR 2,740,489 4/1956 White ct a1 ..181/0.5 WAVES EN TEEEARTH AND DEVIQE 2,760,591 8/1956 White c1 :11... ....118l/0.S 3372.7703/1968 Clynch v ..1s1 0.5

THEREFUR 3,159,233 12/1964 Clynch ct al. ..181/0.5

[75] Inventor: Michel Lavergne, Le Vcsinet,

France [73] Assignee: lnstitut Fraracais du Petrole dies Carlburants etLubrifiants [22] Filed: Aug. 28, 1971) [21] Appl. No.: 67,873

7 Foreign Application Priority Data Sept. 2, 1969 France ..6929979 [52]11.5. C1. .....181/.5 EC, 340/155 SW, 181/.5 NC [51] Int. Cl. ..G01v1/14 [58] Field of Search ..340/15.5 SW; 181/.5 EC

[56] References Cited UNITED STATES PATENTS 3,205,971 9/1965 Clynch..340/15.5 SW

Primary Examiner Benjamin A. Borchelt Assistant Examiner-N. MoskowitzAtrorney-Craig, Antonelli & Hill Method for inducing shear waves in theearth, comprising a generator producing two equal and opposed forces andmeans for transmitting said forces horizontally to vertical sections ofthe ground through the intermediary of springs of different rigidities,whereby said forces become unbalanced and the resultant force istransmitted horizontally to the ground.

ABSTRACT 14 Claims, 1 1 Drawing Figures PATENTEDFEB13 I975 SHEET 2 BF 3FIGJ.

FIG.5

FIG.7

METHOD FOR INDUCING SHEAR WAVES IN THE EARTH AND DEVICE THEREFORAccording to the methods commonly used in seismic prospecting, acousticwaves are transmitted into the earth and propagate through the sub-soil.There are mainly used those waves which propagate through the deepformations by expansion and are reflected by the different sub'soilstrata. These waves are commonly called longitudinal waves and known inthe art as P- type waves.

However other waves which are propagated in depth by distorsion andreflected by the sub-soil formations are also of interest. These waves,called shear waves are of two types:

the shear waves imparting to the particles a horizontally polarizedmotion, called in the art SH waves, and

the shear waves imparting to the particles a vertically polarizedmotion, called in the art SV waves.

The use of shear waves in seismic prospecting may frequently givefurther informations in addition to those derived from the study of thelongitudinal waves, so that in some cases the seismic informations havea greater significance with respect to the surveyed subsoil strata.

Many reasons confirm the interest of making use of the shear waves suchas:

a. their propagation velocity through the sediments is about one half ofthe corresponding velocity of the iongitudinal waves and accordingly,the wave length of the shear wave is about one half of the wave lengthof the longitudinal wave, at the same frequency. The resolution power,i.e., the capability of identifying two subsoil layers at small distancefrom each other is substantially improved, which in some cases is ofhigh interest particularly for detailed surveys of bevelled formationsor stratigraphic traps.

b. the horizontally polarized shear waves (SH waves) exhibit thepeculiar property of being not converted to other types of waves in thepresence of stratigraphic discontinuities, to the extent that suchdiscontinuities are symmetrical with respect to the waves propagationvertical plane. In contrast to the longitudinal (P) waves which undergoa conversion to vertical shear waves (SV) or to the vertical shear waves(SV) which undergo a conversion to longitudinal (P) waves at eachsub-soil interface and when the wave fronts are not parallel to theinterfaces, the SH shear waves are never converted to a wave of adifferent type. The seismograms obtained in oblique incidence onterrains exhibiting many discontinuities are accordingly much simplerwhen using SH waves rather than P or SV waves.

c. with the knowledge of the shear waves propagation velocity, togetherwith the knowledge of the longitudinal waves propagation velocity and ofthe specific gravities of the terrain, it is possible to determinecertain physical parameters of the rocks such as Young's modulus,Poissons coefficient, rigidity modulus, compressibility modulus, suchdetermination being impossible when only the longitudinal wavespropagation velocities are known.

Several methods have been contemplated for transmitting shear waves butthey do not offer the advantages of the method of this invention wherebymuch shorter pulses ofa purer shape are produced.

A first method consists of using horizontal vibrators, imparting to theground alternative horizontal motions by friction (for example U.S. Pat.Nos. 3,159,233 and 3, 286, 783). The inconvenience of this methodresults from the fact that the pulses generated by vibrators are of along duration, usually several seconds. Although they can be contractedat the recording by use of the correlation techniques, it is apparentthat, in some cases, shorter pulses must be generated having for examplea duration ofless than 30 milliseconds.

Another method consists of horizontally propelling a mass against a stopmember fastened to the ground, so as to impart a horizontal forcethereto, (for example US. Pat. No. 2,740,488). However, according tothis method, the mass is suspended from a beam by cables, which presentssome drawbacks. In particular theaction and the reaction forces on theground are not controllable.

It is an object of the present invention to provide a new method fortransmitting shear waves to the earth and an apparatus therefor wherebyshorter pulses of purer shape are generated and the action and reactionforces produced are controllable.

This method consists of producing by means of a generator two equal andopposed forces F and F controlling said forces by means of systemshaving different rigidity coefficients, respectively k, and k so as toobtain opposed and unequal forces F and F which are imparted to theearth along a substantially horizontal direction.

The device for performing this method comprises a generator providingtwo equal and opposed forces F and F means for controlling said forcesand converting the same to opposed and unequal forces F and F and meansfor transmitting said forces F, and F to the earth along a substantiallyhorizontal direction.

This device further comprises frictionless connecting means between thegenerator and the earth.

The invention will be described more in detail with reference to nonlimitative specific embodiments given for illustrative purposes, inconjunction with the accompanying drawings in which:

FIG. 1 illustrates the principle of the method of the invention.

FIG. 2 diagrammatically shows a first embodiment of apparatus forperforming the method of the invention.

FIG. 3 is a diagram obtained by plotting forces F and F' applied to theground along a substantially horizontal direction, versus time.

FIG. 3 is a diagram obtained by plotting the resultant force F, appliedto the ground in a substantially horizontal direction, versus time.

FIG. 4 diagrammatically illustrates the position of the transmitting andreceiving devices in the case of shear waves of the SV type.

FIG. 5 diagrammatically illustrates the position of the transmitting andreceiving devices in the case of shear waves of the SH type,

FIG. 6 diagrammatically shows a second embodiment of the deviceaccording to the invention.

FIG. 7 diagrammatically shows a third embodiment of the device.

FIG. 8 diagrammatically shows a fourth embodiment of the device.

FIG. 9 diagrammatically shows a fifth embodiment of the device.

FIG. 10 diagrammatically shows a seventh embodiment of the device, and,

FIG. 11 diagrammatically shows a seventh embodiment of the device.

The working generator G whose vertical axis passes at point generatessimultaneously a horizontal force F, in the OX direction and ahorizontal force F F in the opposed direction (0 X). The device includesmeans 3 and 5 for controlling respectively the action and reactionforces F and F and rendering them unequals. These means may bedeformable elements with rigidity coefficients k and k acting forrespectively converting the forces F, and F in forces F and F Thedevices further includes elements 2 and 4 for transmitting the forces F.and F respectively in direction OX and (O X). Said forces are impartedto the ground either directly by means of the transmission members 2 and4 applied against ground walls substantially perpendicular to the OXaxis, or through stop members anchored in the ground and placed atdistances x, and x from point 0, respectively in the directions OX and'OX.

The forces F, and F being unequal, a resultant force F, is transmittedto the earth in the form of a seismic pulse.

As a matter of fact the distance x, x of about one meter, is generallysmall as compared to the lengths of the transmitted waves (at leastabout meters) and the impact points x and x of forces F and F may beconsidered as a single point with respect to the scale of thetransmitted seismic waves.

The horizontal force F(t) applied on the earth has accordingly asalgebraic value the sum F,(t) F' (t).'

One of the essential features of this invention is the fact that therigidity k of the control element 3 and the rigidity k ofa differentvalue, of the control element 5,

are so selected that the transfer functions F /F and.

Fz/F be different, whereby there can be obtained forces F,(t) and F (t)which are unequal in absolute value in spite of the equality of theabsolute values of FA!) and F (t).

Consequently the resultant force F(t) F',(t) F I) applied horizontallyonto the ground is not zero and can be selected at will of the desiredshape and magnitude by varying the rigidity coefficients k and k ofelements 3 and 5.

The force generator G may be for example a percussion system, the energysource being pneumatic, hydraulic or electromagnetic, or, if desired anexplosion system, the energy source being a solid, liquid or gaseousexplosive charge, a sparker or a system with an explosive wire or thelike.

A few specific embodiments of a device for performing the method of theinvention are described below by way of non-limitative examples.

According to a first embodiment, illustrated in FIG. 2, the generator Gis, for example, a percussion system comprising a piston actuated from acylinder 1; the force F is transmitted through piston 7 and the force Fthrough cylinder 1 ofthis system.

The generator G is frictionless supported on a carrier member 6, e.g., apneumatic tube. The stresses induced in the earth through the carriermember 6 have substantially no horizontal component since the carrier isfrictionless operated.

By way of example the resultant force F(t) horizontally applied onto theground in the case of the embodiment of FIG. 2, will be determinedhereinafter:

Let

m be the mass of piston 7 x its distance from point 0 in the OXdirection, (.22 its velocity) m a the mass of cylinder 1, x its distancefrom point 0 along axis -XOX (x' its derivative with respect to time,i.e., its velocity) m the mass of pulsing plate 2 x, its distance frompoint 0 along axis XOX ()2, its derivative with respect to time, i.e.,its velocity) m the mass of the pulsing plate 4 x its distance frompoint 0 along axis XOX (x' its derivative with respect to time, i.e.,its velocity) For sake ofsimplification it will be assumed that theconnection between piston 7 and pulsing plate 2 is rigid (k,= izet:

By eliminating i i and )2, between these 5 relationships and having inmind that F F there are obtained forces F and F' in harmonic condition:

It is thus possible to adjust F',, by variation of m and m,, which isequivalent to the variation of the coupling of plate m, with the ground.It is possible to adjust F' by variation of m m I and k The system forapplying the forces to the ground is represented by the transferfunctions F' /F and F /F functions of the frequency.

The transfer function F' /F can be caused to correspond for example to aresonance at high frequency and poorly damped and the transfer functionF /F to a low frequency and well damped resonance.

The resultant force F(t) applied to the earth is then formed ofapositive component, short in time and having a high amplitude,essentially due to F',(t), and a negative component, extending over alonger time, and having a small amplitude, essentially due to F' (t).

By way of illustration the values of functions F' (t) and F (t) will becalculated for the case of a hard ground (small admittance, y, y- O) Thetransfer functions F./F and F' /F, given by relationships (2) and (3)are then simplified to F /F,=l,and 4 a; 1 k 'F! 3 1+ w to It appearsthat F,(t)=F,(t)

In order to obtain F (t), there will be used the Fouriers inversetransform of the transfer function of second order given by relationship(5 Let:

m vk lm be the own pulsation of the system formed by cylinder 1 andspring 5,

n =/(2 w m be the damping coefficient. U( t) the Heaviside scale unit:

* the convolution sign There is obtained:

For n I:

.FIG. 3 diagrammatically shows, by way of example, the magnitude of theforce F' (t) applied to the ground by the shear waves transmitter in theparticular case where the rigidity k of spring 5 isso selected that wlily/"l3 is equal to 62.8, i.e., where the resonance frequency of mass mof cylinder 1 on the spring 5 is equal to Hertz and the dampingcoefficient n is l.

It has been assumed that the force F (t) applied by the generator G wasof the form indicated in the figure, with a time constant selected atabout 5 milliseconds.

F',(t) F,(t) is the force applied by plate 2 onto the ground. It isequal to the force applied by the generator as the ground is supposed tohave a small admittance coefficient 7 0.

F (t) is the reaction force exerted by plate 4 onto the ground throughspring 5.

F(t) F' (t) F' (t) is the horizontal force applied on the ground by thedevice.

The force F'(t) is applied along the direction OX at an applicationpoint which may be considered as the medium point 0 of the device.

The parameters k k m m will be selected in accordance with thefrequencies, so that:

(a)fl, =(w0)/(21r) is lower or equal to the smallest useful frequency ofthe transmission spectrum. For example, in seismic prospecting, wherethe useful band is between 10 and Hertz,f,, may be chosen, as in theexample of FIG. 3, equal to 10 Hertz.

(b)f (w1)/(21r) with (0 V k lm is higher than f without upper limiti.e.,f f

In the case of FIG. 3, for example the selected value off isf this beingachieved very easily by making rigid the connection between piston 7 andplate 2, i.e., by choosing k In practice the upper limit of thetransmitted frequencies is not defined by f but by the own frequencyf ofgenerator G or by the coupling frequency f of the plate 2 with theground having the admittance y,.

The dampening coefficient n 1 /(2w m will be selected high enough foravoiding the so-called singing phenomenon due to a too small damping. Inpractice this coefficient must be higher than 0.3.

In the case, for example, where O.3 n l the higher n, the greater is thedamping.

When n a 1 the damping is total.

The adjustement of n may be achieved by changing the frictioncoefficient 9.

The selection of the OX axis direction depends on the type of shear waveto be transmitted.

for transmitting shear waves of the SV type, the OX axis will beoriented in the direction of the seismic profile on survey, theseismographs being also placed horizontally in the direction of theprofile, so as to receive SV waves.

for transmitting shear waves of the SH type, the OX axis will beoriented in a direction perpendicular to the seismic profile on survey,the seismograph being also placed horizontally in a directionperpendicular to the profile, so as to receive SH waves.

FIG. 4 is a diagrammatic plane view of the assembly of transmitter G andseismographs 5,, S S S, for use in a prospection by means of SV waves.

FIG. 5 is a diagrammatic plane view of the assembly of transmitter G andseismographs S S S S for use in a prospection by means of SH waves.

Other embodiments of the device for performing the invention may becontemplated.

For example, according to a second embodiment, which is a variant of thedevice illustrated in FIG. 2, the frictionless elastic connecting means6 is replaced with a liquid. In this case the cylinder 1 of thegenerator floats on a liquid 9 placed in a vessel 8, as shown in FIG. 6.

The operation of the device in this case is the same as in the case ofFIG. 2.

According to a third embodiment of the device of this invention,illustrated in FIG. 7. the frictionless elastic connection is achievedby roller bearing means. The cylinder 1 of the generator in this caserests on the ground through the intermediary of bearings 10.

According to a fourth embodiment of the shear waves transmitting device,the horizontal forces F' and F are imparted to the ground through theintermediary of plates 11 and 12 covered with spikes.

The plate 12 is integral with a stop member 13 on which piston 7 willstrike. The plate 12 supports the generator cylinder 1 through africtionless system; the cylinder and opposed to piston 7 takes itsbearing on a stop member 14 integral with plate 12, through theintermediary of a spring of a rigidity k or on two stop members 14 and15 through the intermediary of two springs 5 and 3 of respectiverigidities k and k According to a fifth embodiment of the device, whichis a variant of the preceding one, the plate 11 is replaced with a pile16 imbedded in the ground. The pile supports the stop member 13 on whichpiston 7 strikes (FIG. 9).

According to a sixth embodiment, illustrated in FIG. 10, and which is avariant of the preceding embodiment, the cylinder 1 of the generatorrests on the ground through the intermediary of the frictionless elasticconnection system 6 and a plate 17. The forces F and F are imparted tothe ground through two vertical stop members imbedded in the ground, forexample piles l6 and 18 respectively provided with stop members 13 and14. The end of the cylinder 1 which is opposed to piston 7 takes itsbearing during the back motion, on the stop member 14 through a spring5.

Optionally the spring 5 may even be omitted and the connection betweencylinder 1 and stop members 14 made rigid provided there is induced anunsymmetry between the forces F and F imparted to the ground, byintroducing a dissymmetry in the conditions of coupling between piles 16and I8 and the ground: for example the pile 16 is deeply imbedded so asto obtain a high frequency coupling with the ground and the pile 18 isembedded at low depth so as to obtain a low frequency coupling with theground.

It is also possible to obtain such a low frequency coupling by directlylaying cylinder 1 on the loose ground.

A seventh embodiment for the transmission of shear waves by means of agenerator making use of explosive charges is illustrated in FIG. 11.

In this case the energy is generated in the form of a pressure wave bythe explosion, in a chamber 19 containing a liquid of an explosivecharge 20; the liberated force is transmitted to the ground through oneof the walls of the chamber which consists of a deformable membrane 21.I

The generator rests on the ground, for example as in the case of FIG. 8,through the intermediary of a frictionless elastic connection system 6and a plate 12 provided with spikes imbedded in the ground.

The plate 12 being provided with a vertical stop member 14 on the sideof the rigid end portion 22 of the chamber 19 which is opposed to thedeformable wall 21. A spring 15 connects the stop member 14 to this endportion 22. Another'vertical stop member 13, integral with a plate 11,substantially horizontal and fastened to the ground by means of spikes,is placed on the side of the deformable wall 21 of the chamber.

At the time of the explosions the force F, is transmitted to the groundthrough the deformable wall 21 and the stop member 13 whereas the forceF' is transmitted through the end portion 22, the spring 5 and the stopmember 14.

The principle of application of the forces to the ground is the same asin the various preceding cases.

It is clear the previously described device making use of an explosivecharge may be replaced with any other device adapted to generate apressure variation within a chamber containing a liquid, or the liquidof the chamber may be replaced by a gaseous explosive mixture, withoutdeparting from the scope and the spirit of the invention, such changesbeing intended to be within the full range of equivalence of thefollowing claims.

What I claim is:

1. A method for transmitting shear waves to the ground comprisinggenerating two equal and opposed forces, adjusting said two forces byapplying thereto different multiplying coefficient so as to obtain twoopposed and unequal forces, and simultaneously applying the two forcesunequal to the ground along a substantially horizontal direction.

2. A device for transmitting shear waves to the ground comprising agenerator (G) producing two equal and opposed first and second forces (Fand F means for controlling said forces including means for applying tosaid first force (F a first multiplying coefficient (k,) and to saidsecond force (F a second multiplying coefficient (k such that first andsecond opposed and unequal resultant forces (F and F are provided, andmeans for simultaneously transmitting said resultant forces to theground along a substantially horizontal direction.

3. A device according to claim 2 wherein said means for applying saidfirst coefficient (k is a spring (3).

4. A device according to claim 2 wherein said means for applying saidsecond coefficient (k is a spring (5).

5. A device according to claim 2 wherein the generator (G) comprisesessentially a cylinder (1) and a piston (7) movable with respect to eachother.

6. A device according to claim 2 wherein the generator (G) comprisesessentially a closed chamber (19) containing a liquid, said chamberprovided at one end thereof with a deformable wall (21) and at the otherend with a rigid wall (22).

7. A device according to claim 2 further comprising frictionlessconnection means arranged between the generator (G) and the ground forsupporting the generator.

8. A device according to claim 7 wherein said connection means consistsof elastic means (6) for supporting the generator.

9. A device according to claim 7 wherein said connection means consistsof roller means (10) for supporting the generator.

10. A device according to claim 7 wherein said connection means consistsofa liquid mass (9) for supporting the generator.

11. A device according to claim 2 wherein the means for transmittingeach resultant force (F',, F' to the ground consists of a pulsing plate(2, 4) arranged on a substantially vertical wall of the ground.

12. A device according to claim 2 wherein the means for transmittingeach force (F,, F' to the ground consists of a pulsing plate arranged ona substantially vertical stop member (13, 14).

13. A device according to claim 12 wherein each stop member (13, 14) isintegral with a pile (l6, l8) imbedded in the ground, in a substantiallyvertical direction,

14. A device according to claim 12 wherein each 5 stop member (13, 14)is integral with an associated substantially horizontal plate (11, 12)fastened to the ground.

1. A method for transmitting shear waves to the ground comprisinggenerating two equal and opposed forces, adjusting said two forces byapplying thereto different multiplying coefficient so as to obtain twoopposed and unequal forces, and simultaneously applying the two forcesunequal to the ground along a substantially horizontal direction.
 1. Amethod for transmitting shear waves to the ground comprising generatingtwo equal and opposed forces, adjusting said two forces by applyingthereto different multiplying coefficient so as to obtain two opposedand unequal forces, and simultaneously applying the two forces unequalto the ground along a substantially horizontal direction.
 2. A devicefor transmitting shear waves to the ground comprising a generator (G)producing two equal and opposed first and second forces (F1 and F2),means for controlling said forces including means for applying to saidfirst force (F1) a first multiplying coefficient (k1) and to said secondforce (F2) a second multiplying coefficient (k2), such that first andsecond opposed and unequal resultant forces (F''1 and F''2) areprovided, and means for simultaneously transmitting said resultantforces to the ground along a substantially horizontal direction.
 3. Adevice according to claim 2 wherein said means for applying said firstcoefficient (k1) is a spring (3).
 4. A device according to claim 2wherein said means for applying said second coefficient (k2) is a spring(5).
 5. A device according to claim 2 wherein the generator (G)comprises essentially a cylinder (1) and a piston (7) movable withrespect to each other.
 6. A device according to claim 2 wherein thegenerator (G) comprises essentially a closed chamber (19) containing aliquid, said chamber provided at one end thereof with a deformable wall(21) and at the other end with a rigid wall (22).
 7. A device accordingto claim 2 further comprising frictionless connection means arrangedbetween the generator (G) and the ground for supporting the generator.8. A device according to claim 7 wherein said connection means consistsof elastic means (6) for supporting the generator.
 9. A device accordingto claim 7 wherein said connection means consists of roller means (10)for supporting the generator.
 10. A device according to claim 7 whereinsaid connection means consists of a liquid mass (9) for supporting thegenerator.
 11. A device according to claim 2 wherein the means fortransmitting each resultant force (F''1, F''2) to the ground consists ofa pulsing plate (2, 4) arranged on a substantially vertical wall of theground.
 12. A device according to claim 2 wherein the means fortransmitting each force (F''1, F''2) to the ground consists of a pulsingplate arranged on a substantially vertical stop member (13, 14).
 13. Adevice according to claim 12 wherein each stop member (13, 14) isintegral with a pile (16, 18) imbedded in the ground, in a substantiallyvertical direction.